1. Field of the Invention
The present invention relates to an objective lens for a DVD/CD compatible optical pickup, and more particularly to an objective lens for a DVD/CD compatible optical pickup which can improve optical efficiency while minimizing aberration occurring due to different disc thicknesses of different optical storage media.
2. Description of the Related Art
An optical disc is used as a medium for recording information, which enables the recording/reproducing of information without bringing a recording/reproducing head into contact with the recording medium, and also improves recording density.
In order to reproduce information recorded on such an optical disc, a laser beam outputted from an optical pickup as a recording/reproducing head is irradiated to a signal recording layer of the optical disc, and an optical detector mounted on the optical pickup detects a laser beam reflected from the signal recording layer, thereby reading the information.
As an optical disc allowing recording/reproducing of information, there is, for example, an optical disc employing a phase change recording layer that is irradiated with a laser beam to alternate between a crystalline state and an amorphous state.
In order to record information on this kind of optical disc, a laser beam modulated according to a signal of the information to be recorded is irradiated to the signal recording layer.
There is a DVD (Digital Video Disc or Digital Versatile Disc), as one of the optical discs that use a phase change recording layer to permit recording/reproducing of information. The DVD has a recording density greatly improved over a CD (Compact Disc), an optical disc in wide use. While the CD has a recording track pitch of 1.6 μm, the DVD has a recording track pitch of 0.74 μm, thereby achieving high recording density.
In order to record information, by laser beam irradiation, on such an optical disc allowing high-density information recording, it is required to form a much smaller beam spot on the signal recording layer of the optical disc.
The spot diameter of a laser beam is proportional to the design wavelength of the laser beam, and inversely proportional to the numerical aperture (NA) of an objective lens that focuses the laser beam. Therefore, for achieving high recording density of an optical disc, it is required to increase the numerical aperture of the objective lens and decrease the wavelength of the laser beam.
In the meantime, a DVDP and a DVD-ROM is attracting attention in imaging/acoustic fields, as a device capable of performing high density recording/reproducing. For compatibility, an optical pickup device employed in the DVDP must be able to record and/or reproduce information when it adopts not only the DVD but also CD family such as CD, CD-R (CD-Recordable), CD-I and CD-G, as a recording medium.
However, the DVD has been standardized in a different thickness from the CD family, due to mechanical tolerance for disc tilt and the numerical aperture of an objective lens. The thickness of the conventional CD family is 1.2 mm, while the thickness of the DVD is 0.6 mm. Since the thickness of the CD family is different from that of the DVD, if an optical pickup device for the DVD is applied to the CD family, the thickness difference causes spherical aberration. This spherical aberration leads to a failure to obtain enough optical intensity to record an information signal, or lowers the quality of reproduced signals.
In addition, regarding the wavelength of a light source for reproducing, the DVD has also been standardized in a different wavelength range from the CD family. That is, the wavelength of the reproducing light source for the conventional CD family is about 780 nm, whereas the wavelength for the DVD is about 650 nm.
Due to the different standardizations, it is impossible for a conventional CDP to reproduce information recorded on the DVD. Thus, there is a need to develop the optical pickup device for DVD, which must also be compatible with the conventional CD family.
In the prior art, a ring-shaped optical disc device has been proposed, which can reproduce different kinds of optical discs such as CD and DVD.
Both the CD and the DVD (hereinafter, collectively referred to as an “optical disc”) employ a transparent substrate that has an information recording surface at one side thereof. The optical disc is formed by attaching two such transparent substrates to each other so that the respective information recording layers face each other, or by attaching such a transparent substrate to a protective substrate so that an information recording layer thereof faces the protective substrate.
In order to reproduce an information signal stored in the optical disc having such a configuration, it is required for an optical disc device to focus a laser beam from a light source onto the information recording surface of the optical disc via the transparent substrate.
The wavelength of a laser beam used for the CD is different from that of the DVD, as described below. The optical disc device uses an objective lens in order to focus a laser beam.
As described above, the thickness of a transparent substrate used for the CD is 1.2 mm, whereas the thickness of a transparent substrate used for the DVD is 0.6 mm. The thickness of the transparent substrate on which the information recording surface is formed varies depending on the kind of optical disc or the wavelength of the laser beam.
For an optical disc device for reproducing different kinds of optical discs, it is required to focus a laser beam onto the information recording surface even if the thickness of the transparent substrate varies depending on the kind of optical disc.
In new optical disc devices recently suggested, it has been proposed to use a blue laser having a wavelength of about 400 nm in order to perform reproduction of information. It is thus expected that the optical disc device can use the CD for downward compatibility and the DVD now in use, and can also use such a new optical disc.
It is considered that such a ring-shaped optical disc device is classified into the following two types. In one type, for each kind of optical disc, an individual objective lens is installed in a pickup, and the objective lenses are exchanged according to the kind of optical disc to be used. In the other type, an individual pickup is installed for each kind of optical disc, and the pickups are exchanged according to the kind of optical disc to be used.
However, in order to realize a low cost, small size device, it is desirable to be able to use the same objective lens for any kind of optical disc.
One typical example of such a kind of objective lens can be seen in Japanese Patent Publication No. Hei9-145995. The objective lens described in this document has a lens surface divided into three or more concentric ring-shaped lens surfaces radially arranged, in which the refractive power of one ring-shaped lens surface of one color is different from that of another ring-shaped lens surface of another color.
Said one ring-shaped lens surface of one color allows a laser beam of a wavelength to be focused onto an information recording surface of a thin optical disc of 0.6 mm thickness (for example, DVD). On the other hand, said another ring-shaped lens surface of another color allows a laser beam of the same wavelength to be focused onto an information recording surface of a thick optical disc of 1.2 mm (for example, CD).
Another typical example can be seen in Japanese Patent Publication No. 2000-81566 (or U.S. Pat. No. 6,118,594). This document has disclosed an optical disc device that uses a laser beam of a short wavelength (635 nm or 650 nm) for the thin transparent substrate (DVD), and uses a laser beam of a long wavelength (780 nm) for the thick transparent substrate (CD). This optical disc device has an objective lens commonly used for the two laser beams.
This objective lens has a refractive lens surface having a positive refractive power, on which fine concentric ring-shaped steps are closely formed, thereby obtaining a diffractive lens structure. This diffractive lens structure is designed to focus a diffractive laser beam of a short wavelength onto the information recording surface of a thin transparent substrate (DVD), and to focus a diffractive laser beam of a long wavelength onto the information recording surface of a thick transparent substrate (CD).
The diffractive lens structure is also designed to focus a diffractive beam of the same diffractive order onto the information recording surface, no matter what kind of diffractive beam it is. The reason why a laser beam of a short wavelength is used for the DVD is because the recording density of the DVD is higher than that of the CD and it is thus required to throttle the beam spot to be smaller. As is well known, the size of an optical spot is proportional to the wavelength, and inversely proportional to the numerical aperture (NA).
A different ring-shaped objective lens has been proposed in Japanese Patent Publication No. 2001-51192, which has a phase-compensated lens structure in which ring-shaped phase shifters are formed on a surface of the lens. Firstly, in this objective lens, a lens surface, which allows removal of wavefront aberration due to a laser beam of a wavelength λ1 (640 nm) used for the DVD, is defined as a reference lens surface. The surface of this objective lens is divided into a plurality of ring-shaped refractive surfaces radially arranged. Each ring-shaped refractive surface is formed to have a predetermined step depth from the reference lens surface (the i-th step depth from the center of the lens is denoted by “di”). Each refractive surface having a step depth (di) allows a laser beam for the DVD to be phase-shifted by an integral multiple (mi) of the wavelength (λ1) with respect to the reference lens surface, thereby reducing the wavefront aberration of the CD family.
Since it allows an objective lens to be commonly used for both the DVD and the CD, all the prior art described above has no need to provide means for exchanging an individual member, including an objective lens, used for each of the DVD and the CD, so it is advantageous in terms of cost and has a simpler configuration.
However, the prior art described above has the following disadvantages. First, the objective lens of Japanese Patent Publication No. Hei9-145995 uses different ring-shaped lens surfaces for the DVD and the CD, so it has many portions invalid for an incident laser beam, and thus has very low optical utilization efficiency.
In addition, the objective lens of Japanese Patent Publication No. 2000-81566 (or U.S. Pat. No. 6,118,594) uses diffractive beams obtained by the diffractive lens structure, so it is impossible to simultaneously obtain 100% diffractive efficiency for each of different wavelengths.
In order for the lens surface to have the diffractive lens structure, it is required to form minute steps on the lens surface. But, this is liable to be affected by manufacturing errors. An error at the time of designing the diffractive lens structure also causes deterioration of the diffractive efficiency.
Due to such diffractive efficiency deterioration or inherent impossibility of obtaining 100% diffractive efficiency, it is impossible to focus every incident beam onto the information recording surface formed on the transparent substrate of the optical disc, which causes an optical loss.
In addition, as described above, the ring-shaped objective lens disclosed in Japanese Patent Publication No. 2001-51192 has a phase-compensated lens structure for providing high optical utilization efficiency. A lens surface, which allows removal of wavefront aberration due to laser beams used for the DVD, is defined as a reference lens surface, and the surface of the lens is divided into a plurality of ring-shaped refractive surfaces radially arranged. In order to reduce wavefront aberration of laser beams for the CD, a ring-shaped refractive surface is formed to be recessed from the reference lens surface by a step depth (di), which corresponds to an integral multiple (mi) of the wavelength (λ1) of laser beams for the DVD. However, since it is inherently based on the DVD, it is impossible for only the step formation to sufficiently reduce the wavefront aberration for laser beams for the CD.